The present invention relates to improvements in heat exchangers, and more particularly, to improvements in rodbaffle heat exchangers for preventing undesirable acoustical vibrations from occurring in such heat exchangers. It is known that acoustic resonance may be encountered in shell and tube heat exchangers having a gaseous or two-phase fluid flowing on the shell side when the vortex shedding frequency approaches the acoustic resonance frequency of the flowing gaseous medium. The acoustic resonance frequency is directly proportional to the sonic velocity of the flowing medium and is inversely proportional to the shell diameter. Acoustic resonance oscillations are frequently characterized as standing waves extending across the shell diameter and occurring perpendicular to both the direction of flow and the longitudinal axis of the tube bundle. In addition to objectionable noise, acoustic resonance can also produce damage to tube bundles when the acoustic resonance frequency approaches the natural frequency of the tubes.
There are a number of possible methods for correcting this undesirable condition including changing the flow rate of the fluid flow on the shell side. However, this involves a reduction in flow and is accompanied by reduction in the convective heat transfer. Similarly, the removal of several tubes is possible, but this may also cause unacceptable reduction of heat transfer, and is extremely difficult and expensive when an exchanger is already in operation in the field. Another method involves a construction using irregular lateral spacing of the tubes; however the type of exchangers commonly known as rodbaffle exchangers are much more efficient if such irregular lateral spacing is not practiced. In addition, the prevention of premature failure of tubes due to mechanical stress is taught in U.S. Pat. No. 4,136,736 which is assigned to the assignee of the present application. As used in that patent, the term "baffle" refers to an annular ring to which the ends of a plurality of rods are connected; hence the term "rodbaffle". Many heat exchangers of this type have been constructed and successfully operated without damage due to vibrations of the tubes. However, under certain operating conditions it is possible that the standing waves of acoustical vibrations mentioned above may exist in such exchangers, such that, the vibration problem may not always be completely prevented by the rodbaffles of the above mentioned patent.
The use of flow-directing baffles in heat exchangers is generally discussed in "Petroleum Refining Engineering" 2nd ed, 1941, by W. L. Nelson published by McGraw-Hill Book Company, Inc. on pages 393-395. Such baffles are laterally extending plates, perpendicular to the axis of the ouier shell, and they extend into the gas flow on the shell side of the heat exchanger in alternate projections from either side of the shell so as to form a generally serpentine flow path. In other figures, the baffles are shown as having a spiral configuration so as to cause an overall spiral or helical flow of the fluid medium. In these baffles, holes are provided through which the tubes of the exchanger are passed, and the holes may be made larger than the outer diameter of the tubes such that some fluid is forced to "scour" the wall of the tube at the baffle location.
More recently, an article entitled "Acoustic Vibrations in Tubular Heat Exchangers" by E. A. Barrington appeared in Volume 69, No. 7 of Chemical Engineering Progress magazine. In this article, the author describes the acoustic vibrations in significant detail, and suggests the use of one or more solid, one-piece baffles extending longitudinally along the axial length of the heat exchanger in order to break up or prevent the occurrence of acoustical standing waves. While such longitudinally extending baffles may solve the problem of acoustical vibrations in the plate baffle type of exchangers to which the author refers, it is not possible or practical to have one or more one-piece, continuous baffles extending the full length of a rodbaffle heat exchanger since such a baffle would cause severe maldistribution of the fluid flow on the shell side. Accordingly, the solution proposed in this article is not adaptable to the acoustical problem should it occur in heat exchangers of the rodbaffle type.